1. Field of the Invention
This invention relates to a suspension system for a vehicle, and, in particular to an active suspension system.
2. Description of the Related Art
A conventional active suspension is disclosed in, for example, Japanese Laid-Open Patent No. 62-96113 published on May 2, 1987, European Patent Application published on Aug. 1, 1984 under publication number 0114757, and U.S. Pat. No. 4,371,182, issued on Feb. 1, 1983. In active suspensions as disclosed in the publications enumerated above, a cylinder device is provided for each vehicle wheel member between a vehicle boby member and a vehicle wheel member. By controlling a supply of hydraulic fluid to the cylinder device, the hydraulic fluid in the amount of cylinder device can be changed so that the vehicle suspension characteristics are changed.
In the known active suspension systems, a vehicle height displacement is detected and a supply of hydraulic fluid to the cylinder device is controlled to get a target value of the vehicle height displacement. This is known as a vehicle height displacement control.
It should, however, be noted that this type of vehicle height displacement control, based on the detected vehicle height displacement, is not responsive. Thus, the control cannot accomplish a desirable vehicle height displacement control as the vehicle makes a quick movement.
As a result, a vehicle attitude may not be maintained properly. In addition, riding comfort is deteriorated.
In order to solve these problems, it is proposed that vehicle height displacement change rate be detected to control the hydraulic fluid introduced into the cylinder device to suppress the vehicle height displacement change rate ( vehicle height change rate control).
The vehicle height displacement control and the vehicle height change rate control are carried out independently. Therefore, both of the controls must be coordinated with each other for improving the suspension properties.